Since Michael Phelps and other swimmers are able to spend 5 hours a day swimming, maybe doing that underwater running as a training supplement is the secret to long distance running.
I think even top marathon runners only train about 2 hours a day, and don't burn burn the same number of calories as Phelps. I haven't heard of a diet as ridiculous as Phelp's
I know the pool trains different muscles, but as a supplement, and to work on cardio conditioning, it could help.
Just an idea.